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Decomposition  of  Hydrated  Ammonium  Salts. 


William  M.  Dehn  and  Edward  CX  Heuse, 


\ s O^p  Q 3r  CL,  ft, 


5V).3 

HVS 

pX'YA 


[Reprinted  from  the  Journal  of  the  American  Chemical  Society 
Vol.  XXIX,  No.  8,  August,  1907]. 

[Contribution  from  the  Chemical  Laboratory  of  the  University 

of  Illinois.] 

DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS. 

By  William  M.  Dehn  and  Edward  O.  Heuse. 

Received  May  31,  1907. 

Und£r  the  influence  of  rising  temperatures,  diammonium  oxalate1  de- 
composes as  follows: 

I.1  (C00NH4)2.H20  = (COONH4)2  + H20 

II.2  (COONH4)2.H20  = (COOH)2  + H20  + 2NH3 

III. 3  (C00NH4)2.H20  = HCOOH  + C02  + 2NH3  + H20 

IV. 4  (C00NH4)2.H20  = (CONH2)2  + 3h2o 

V.5  (C00NH4)2.H20  = C02  + CO  + 2NH3  + 2H20 

VI.6  (C00NH4)2.H20  ==  HCN  + C02  + NH,  + 3H20 

VII.7  (C00NH4)2.H20  = c2n2  + 5h2o 
Some  of  these  reactions  take  place  at  approximately  the  same  tempera- 
tures ; others  only  at  successively  higher  temperatures.  That  equation  I 
represents  the  initial  decomposition  is  established  with  certainty1;  the 
end-products  at  high  temperature  are  shown  to  be  largely  cyanogen  and 
water.  It  wb  e observed  that  only  equations  I and  II  represent 
reversible  reactions  which  are  rapid  and  complete ; the  products  of  reac- 
tion III  condense  to  ammonium  formate  and  ammonium  bicarbonate; 
some  of  the  products  of  reaction  V and  VI  condense  to  ammonium  carbon- 
ate and  ammonium  bicarbonate ; reaction  IV  and  VII8  are  practically  non- 
v • reversible. 

1 Dupre,  Analyst,  30,  266;  Ber. , 18,  i394;Gillot,  Bull.  Acad.  Roy.  Belg. , 1900,  744. 

2 Gillot;  Gay  Lussac,  Ann.  chim.  phys.,  46,  218. 

3 Turner,  Schweigg,  Jour.  62,  444;  Pogg.  Ann.,  24,  166.  Dumas,  Ann.  chim. 
phys.,  44,  129  (1830). 

4 Ibid,  54,  240. 

5 Dumas;  Gay  Lussac;  Lorin,  Compt.  rend.,  82,  750. 

6 Dumas. 

7 Dumas;  Michael,  Ber.  28,  1632. 

8 Peleuse  and  Richardson  (Ann.  26,  63)  show  that  water  and  cyanogen  yield 
ammonium  oxalate.  Zellet  (Monatshefte  14,  224)  finds  that  when  cyanogen  is  heated 
with  water  at  ioo°,  he  obtains  oxalic,  hydrocyanic  and  azulmic  acids,  urea,  carbon 
dioxide,  and  ammonia. 

1 


19342 

f 


1 138 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


From  consideration  not  only  of  the  number  of  these  possible  reactions 
but  also  of  the  diversity  of  the  products  formed,  it  may  be  supposed  that 
the  decomposition  of  this  simple  salt  involves  a hopeless  complexity ; how- 
ever, the  following  studies  seem  to  indicate  that  the  order  of  successive 
decompositions  is  largely  as  shown  in  the  equations  : 

(C00NH4)2.H20  = (COONH4)2  + h2o 
(COONH4)2  = (CONH2)2  + 2H20 
(CONH2)2  = C2N2  + 2H20 

Samples  of  pure  diammonium  oxalate  in  open  crucibles  were  heated  in 
air  baths  whose  temperatures  were  held  constant  during  one  hour.  The 
total  loss  in  weight  and  the  residual  ammonia  were  determined  .in  each  ex- 
periment with  the  following  results : 

Loss  per  cent  of 


Experiment 

Temperature 

Total 

Ammonia 

Water1 

I 

8o° 

9-51 

9-5i 

2 

95° 

12.17 

12.17 

3 

1180 

13-17 

I3-J7 

4 

143° 

14.02 

14.02 

5 

153° 

15-42 

o-33 

15.09 

6 

1680 

63.78 

6.75 

56.03 

7 

0 

00 

!>. 

78.65 

13-86 

64.73 

8 

193° 

87.89 

13.92 

74-03 

9 

243° 

89.21 

18.95 

70.26 

These  experiments  show  that  when  dry  diammonium  oxalate  is  heated : 

1.  It  evolves  one  molecule  of  water  below  10002  (Equation  1)  ; and,  to 
150°  at  least,  decomposes  according  to  Equation  IV. 

2.  Below  1 68°  the  loss  of  water  is  even  greater  than  that  represented  by 
equation  IV  (38.02  per  cent.)  ; hence  oxamide  is  largely  formed  at  these 
temperatures. 

3.  At  150°  ammonia  begins3  to  be  evolved  (Equation  II)  and  at  higher 
temperatures  it  continues  to  be  evolved  or  else  the  substance  sublimes. 

Since  oxamide  sublimes  but,  as  shown  below,  does  not  decompose  into 
cyanogen  and  water  (Equation  VII)  below  280°,  it  may  be  concluded  that 
the  lower  temperatures  represent  only  two  main  decompositions  (I  and 
IV).  Efforts  were  made  to  confirm  this  by  vapor  pressure  curves. 

1 And  other  products  at  higher  temperature  ; the  total  per  cent,  of  water  is 
63.37  per  cent. 

2 One  molecule  of  water  represents  12.67  per  cent. 

3 Gillot  (/.  Chem.  Soc .,  1901,  A 118)  shows  that  ammonia  is  completely  hydro- 
lyzed and  expelled  from  boiling  solutions  of  diammonium  oxalate. 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


139 


Vapor  Pressures  of  Diammonium  Oxalate. 


Temp. 

Pressure 

Temp. 

Pressure 

71 

II 

145 

2134 

74 

19 

156 

2748 

82 

53 

l6l 

3144 

85 

58 

168 

3877 

90 

72 

171 

4319 

95 

78 

176 

4821 

98 

85 

180 

5233 

hi 

3H 

182 

5546 

121 

597 

187 

6084 

122 

827 

195 

7103 

126 

999 

197 

7326 

131 

1271 

200 

7682 

138 

1616 

205 

8219 

141 

1815 

210 

8818 

Vapor  Pressures  of  Oxamide. 


Temp. 

Pressure 

Temp. 

Pressure 

2651 

45 

293 

I596 

270 

71 

294 

1911 

274 

86 

294-5 

2115 

277 

118 

295 

2157 

283 

241 

295-5 

2301 

290 

278 

296.5 

2412 

291 

1182 

297 

2536 

292 

1383 

297.5 

27522 

It  will  be  observed  that  the  vapor  pressure  curve  of  diammonium  oxa- 
late is  represented  by  three  distinct  segments.  Segment  A unquestion- 
ably represents  the  partial  aqueous  decomposition;  segment  B evidently 
represents  the  elimination  of  the  molecule  of  water  of  crystallization 
(equation  I)  ; and  segment  C represents  the  decomposition  into  oxamide 
and  probably  the  simultaneous  decomposition  represented  by  equation  II, 
III,  V and  VI.  That  reaction  VII  does  not  take  place  below  290°  is  suffi- 
ciently indicated  by  the  curve  of  oxamide. 

Monoammonium  Oxalate. 

This  salt,  prepared  by  the  methods  of  Nichols3  and  Walden4,  was  found 
to  be  pure  NH4HC2H4.H20.  It  is  reported  that  when  heated,  this  salt  is 
stable  to  70  0 ; at  higher  temperatures,  it  begins  to  lose  its  water  of  crys- 
tallization5; at  140°  it  forms  oxamic  acid6;  at  more  elevated  temperatures, 
it  yields  carbon  dioxide,  carbon  monoxide,  formic  acid  and  oxamide  ; fin- 
ally, it  expels  hydrocyanic  acid  and  ammonium  carbonate;  the  residue 
contains  oxamic  acid  and  oximide  . 

When  samples  of  the  salt  were  heated  in’  open  crucibles  in  the  manner 
indicated  above,  the  following  data  were  obtained : 

1 Sublimation  was  observed  at  this  temperature. 

2 The  non-reversible  pressure  was  equal  to  1182  mm.;  a large  quantity  of  cyan- 
ogen was  found. 

3 Chem.  News,  22,  14. 

4 Am.  Ch.  J.,  34,  147. 

5 Balard,  Ann.  chim.  phys.,  (3)  4,  94;  Ann.,  42,  197. 

6 Ost.  and  Mente,  Ber.,  19,  3229. 


140 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


Plate,  i 


94 

105 

115 

134 

155 

169 

183 

200 

225 


2.68 

7- 47 

8- 54 

9- °9 

10.99 

n. 51 

48.36 

88.51 

99.64 


0.13 
0.30 
1.03 
1. 71 
8.98 
12.24 


2.68 

7- 47 

8- 54 

8.96 

10.69 

11.48 

46.65 

79-53 

S7.40 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


II4I 


It  is  seen  that  monoammonium  oxalate : 

1 . Is  stable  to  750, 

2.  Parts  with  its  molecule  of  water  of  crystallization  (1440  per  cent.) 
below  170°. 

3.  Loses  two  other  molecules  of  water  at  183°  and  simultaneously  incurs 
a secondary  decomposition  or  sublimes. 

Therefore  the  main  successive  decompositions  are  probably  indicated 
by  the  equations : 

NH4HC204.H20  = nh4hc2o4  + h2o 
NH4HC204  = HOOCCONH,  + H20 

hoocconh2  — cov  + h2o 

I >NH 


CO 


The  above  data  of  decomposition  are  closely  confirmed  by  the  vapor 
pressures  of  the  substance. 


Temperature 

Pressure 

Temperature 

Pressure 

8l 

7-7 

145 

2131 

88 

T5-9 

150 

2370 

95-8 

50.2 

155 

2682 

106 

469 

160 

2964 

no 

686 

165 

3096 

115 

875 

170 

3268 

12? 

1102 

175 

4009 

125 

1230 

176 

5862 

130 

1420 

180 

7427 

135 

1623 

182 

8159 

140 

1840 

185 

9846 

It  will  be  observed  (see  plate  I)  that  (1)  the  general  form  of  the  two 
curves  are  much  alike,  (2)  the  molecule  of  water  in  each  salt  is  com- 
pletely eliminated  below  100- 170°,  and  (3)  the  upper  segments  represent 
the  second  stages  of  decomposition. 

Decompositions  of  the  above  organic  compounds  indicate  that  the  ini- 
tial and  predominating  reactions  involve  the  expulsion  of  water ; and  that 
simultaneously,  particularly  at  higher  temperatures,  secondary  reactions 
indicated  by  the  dissociation  of  ammonia,  are  involved.  It  was  hoped  that 
studies  of  inorganic  hydrated  ammonium  salts,  along  the  lines  indicated 
above,  would  lead  to  a more  intimate  knowledge  of  water  of  crystallization 
and  of  the  structure  of  hydrated  salts.  That  this  hope  has  been  partially 
realized  is  evidenced  by  the  following  studies. 

It  was  found,  for  instance,  that  certain  hydrated  ammonium  salts  de- 
compose so  as  to  yield  both  water  and  ammonia  at  most  temperatures 
above  the  initial  temperature  of  decomposition.  Studies  of  the  rate  of 
expulsion  of  water  and  ammonia  have  shown  that  abundant  yields  of  am- 
monia usually  accompany  the  largest  yields  of  water;  and,  though  the 
last  trace  of  ammonia  is  given  off  only  with  the  last  trace  of  water,  it  is 
given  off  simultaneously  with  it  at  most  of  the  lower  temperatures.  In 


1 142 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


other  words,  curves  of  expulsion  of  ammonia,  as  well  as  of  water,  extend 
from  the  temperatures  of  initial  decomposition  to  those  of  complete  de- 
composition; consequently  tracing  the  course  of  ammonia  through  the 
composite  decompositions  leads  to  knowledge  of  the  respective  individual 
decompositions  and,  as  will  be  shown,  throws  light  upon  individual  struc- 
tures in  the  complete  structure.  For  instance,  suppose  it  can  be  shown 
that  highly  polyhydrated  ammonium  salts  are  largely  decomposed  below 
ioo°,  while  the  residues  of  ammonia  and  of  “ water  of  composition”  are 
completely  expelled  only  at  considerably  higher  temperatures,  it  may  then 
be  concluded  that  the  union  of  ammonia  resembles  more  closely  the  union 
of  “water  of  composition”  than  the  union  of  “water  of  crystallization." 
Again  suppose  it  can  be  shown  that  ammonia  is  given  off  at  all  lower  tem- 
peratures, it  may  also  be  concluded  that  both  “water  of  crystallisation”  and 
“ water  of  composition”  are  given  off  at  all  of  these  lower  temperatures. 

In  respect  to  the  methods  used  to  differentiate  the  respective  dissocia- 
tions, it  has  been  found  that  vapor  pressure  curves  {vide  plate  VII ) are 
not  necessarily  indicative  of  the  qualitative  decompositions  of  compounds ; 
in  the  case  of  polyhydrated  salts  they  area  measurement  only  of  the  com- 
posite effect  of  a number  of  co-temporaneous  dissociations.  For  instance 
if  each  molecule  of  water  and  ammonia  in  the  original  compound  has  a 
definite  vapor  pressure  for  each  temperature,  it  may  easily  be  seen  that  the 
resultants  of  their  pressures  may  so  blend  as  to  indicate  no  definite  breaks 
in  the  vapor  pressure  curve,  therefore,  recognition  of  points  of  decomposi- 
tion may  fail  entirely  when  only  vapor  pressure  curves  are  studied.  For 
this  reason  other  methods  of  investigation  have  been  employed. 

Decomposition  of  Inorganic  Salts. 

Various  investigators1  have  represented  partially  dehydrated  salts,  for 
instance,  hydrated  ammonium  salts,  by  very  contradictory  and,  as  shown 
below,  by  very  erroneous  formulas.  We  find  in  the  periodicals  and  the 
text-books  that  free  use  is  made  of  formulas : 

(NH4MgAs04)2.H20  and  (NH4MgP04)2.H20 
to  represent  ammonium  magnesium  arsenate  and  ammonium  magnesium 
phosphate  dehydrated  at  100-110°:  Though  most  investigators  agree  that 
the  composition  of  ammonium  magnesium  arsenate  at  ordinary  temperature 
is  NH4MgAs046H20,  a considerable  difference  of  opinion  as  to  its  composi- 
tion at  temperatures  between  98°  and  ioo°  is  expressed.  For  instance  Bun- 
sen2 concludes  that  nearly  of  a molecule  of  water  is  held  at  98°.  Rose3, 
Puller4,  Field5,  and  Lefevre6,  affirm  that  exactly  y2  mol.  HsO  is  retained 

1 Wach,  Schweigger’s  J.  Chem.  Physik,  59,  288;  Rose,  Z.  anal.  Chem.,  1,  417; 
Ann.  Physik.,  76,  20;  Z.  anorg.  Chem.,  23,  146.  Puller,  Z.  anal.  Chem.,  10,  68. 

2 Ann.  Pharm.,  192,  311. 

3 Z.  anal.  Chem.,  1,  417. 

4 Ibid,  10,  68. 

5 Jahrsb.,  I858,  170. 

6 Ann.  chim.  phys.,  (6)  27,  55. 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


1143 


by  the  salt  when  it  is  dried  at  ioo°  on  the  water  bath  ; Fuller  holds  that  it 
is  practically  dehydrated  at  103°  ; and  Bunsen1  further  states  that  is  com- 
pletely dehydrated  at  104.5  °.  These  data  do  not  appear  to  be  particularly 
discordant;  but  in  view  of  the  fact,  as  shown  in  this  research,  that  3-4 
per  cent,  of  ammonia — equivalent  in  weight  to  about  y2  mol.  H20 — are 
lost  at  these  temperatures,  none  of  the  conclusions  drawn  are  correct. 
For  when  the  salt  is  dried  at  these  temperatures  less  ammonia  and  more 
water  are  present  than  are  represented  by  the  formula  (NH4MgAs04)2.- 
IT20.  A more  correct  representation  would  be  a mixture  in  equal  pro- 
portions of  HMgAs04.H20  and  NH4MgAs04.H20.  However,  even  this 
formulation  will  be  shown  to  be  incorrect,  for  one  conclusion  of  these 
studies  is  that  no  definite  formula  can  be  given  to  many  hydrated  ammon- 
ium salts  dried  at  temperatures  between  ^.o°-2oo° . 

This  is  clearly  illustrated  by  data  obtained  on  heating  samples  of  these 
salts  at  definite  intervals  of  temperature  for  equal  lengths  of  time  and 
determining  both  the  total  loss  in  weight  sustained  and  also  the  weight 
of  ammonia  evolved.  The  salts,  contained  and  weighed  in  U-shaped 
tubes,  were  heated  in  baths  controlled  by  thermostats,  while  air,  dried 
and  freed  from  carbon  dioxide,  was  passed  continuously  through  the 
tubes  and  into  flasks  containing  standard  sulphuric  acid.  The  total  loss 
of  weight  in  the  U-tubes  represented,  of  course,  the  loss  of  both  water 
and  ammonia  ; this  weight,  less  than  the  weight  of  ammonia,  determined 
by  titration,  gave  the  loss  of  water. 

It  was  found  that  quite  different  results  were  obtained  when  we  varied 
the  following  conditions  : 1.  Temperature.  2.  Time.  3.  Kind  of  dry- 

ing gas.  4.  Quantity  of  drying  gas.  5.  Size  of  salt  crystals.  6.  Pres- 
ervation of  salt  crystals.  7.  Manner  of  heating. 

The  effect  of  temperature  is  the  most  important  and  it  was  on  temper- 
ature as  a basis  that  the  following  studies  were  made. 

The  influence  of  time  was  soon  found  to  be  a very  disturbing  factor, 
for  these  salts  do  not  dry  to  definite  composition,  therefore  briefer  or 
longer  desiccation  gave  very  widely  different  per  cents,  of  decomposition. 

This  is  seen  in  the  following  table  : 


Substance  Weight  Loss  Loss  per  cent.  Time  Temp. 

NH4MgP04.6H,0 0.7473  0.0423  5.66  4 70° 

NH4MgP04.6H20 0.4636  0.1658  35.76  40  70° 

HNaNH4P04.4H20  0.8016  0.1178  14.70  4 76° 

HNaNH4P04.4H20 4.7094  1.0784  22.89  57  75° 

NH4MgAs04.6H20 0.7941  0.0136  0.35  4 50° 

NH4MgAs04.6H20 0.7958  0.0372  4.67  20  50° 

NH4MgAs04.6H20 0.2920  0.1108  37-95  4 no0 

NH4MgAs04.6H20 1.1127  0.4556  39.61  40  no° 


It  will  be  observed  here  that  heating  for  four  hours  invariably  gave  lower 
per  cents,  of  decomposition  than  when  heating  for  20-57  hours.  An  ex- 
1 loc.  cit. 


ii44 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


planation  of  these  great  differences  of  results  on  short  and  protracted 
heating  is  conceivable  when  one  recalls  that  “water  of  crystallization”  is 
more  easily  expelled  than  “water  of  composition”.  The  former  is  usual- 
ly eliminated  at  temperatures  below  ioo° ; the  latter,  at  temperatures 
above  ioo°.  By  protracted  heating  at  low  temperatures,  however, 
“water  of  composition”  may  be  removed  completely,  therefore  too  pro- 
longed heating  at  low  temperature  does  not  reveal  the  normal  decompo- 
sition at  these  temperatures.  On  the  other  hand  too  brief  a heating  at 
low  temperatures  does  not  insure  complete  removal  of  the  decomposition 
products.  It  was  to  avoid  on  the  one  hand  incomplete  dehydration,  and 
on  the  other  excessive  secondary  decomposition  that  periods  of  4-7  hours 
heating  were  finally  chosen. 

It  was  found,  moreover,  that  heating  the  salts  progressively,  that  is 
heating  the  same  sample  to  successively  higher  temperatures,  did  not 
yield  the  proper  results,  for,  on  comparing  the  per  cents,  of  decompo- 
sition obtained  by  heating  different  samples  at  the  respective  tempera- 
tures, very  different  results  were  obtained. 

The  method  of  heating  individual  samples  at  different  temperatures  for 
the  same  lengths  of  time  was  adopted,  in  preference  to  heating  the  same 
sample  successively  to  higher  temperatures,  for  the  reason  that  the 
former  method  really  eliminates  the  element  of  time  and  thus  minimizes 
secondary  decompositions.  For  instance  when  a sample  of  a salt  is 
heated  at  65°  for  four  hours  and  then  another  sample  of  the  same  salt  is 
heated  at  70°  for  four  hours,  all  other  conditions  remaining  the  same, 
the  difference  of  effect  is  the  result  of  temperature  alone.  A further 
reason  for  employing  the  method  of  separate  samples  for  each  interval  of 
temperature,  was  to  avoid  the  accumulative  errors  of  analysis  involved 
in  the  other  method. 

The  effect  of  using  different  gases  to  carry  off  the  decomposi- 
tion products  may  be  seen  in  the  use  of  hydrogen  and  of  air. 
In  the  following  table,  the  data  were  obtained  on  heating  microcosmic 
salt  for  periods  of  four  hours  each. 


Temperature 

Per  cent. 

Carrier 

50° 

0.35 

Hydrogen 

50° 

0.60 

Air 

5o° 

0-59 

Air 

76° 

9.64 

Hydrogen 

75° 

16.35 

Air 

The  results  here  indicate  that  hydrogen  is  not  so  efficient  a carrier  as  air, 
and  this  undoubtedly  is  owing  to  the  fact  that  it  possesses  a much  more 
rapid  rate  of  diffusion. 

The  effect  of  speed , or  rather  the  quantity,  of  carrying  gas,  is  seen  in 
the  following  experiment.  Dry  air  was  passed  for  27  hours  over  4.4334 
grams  of  NH4MgAs04.6H,0  ; it  lost  0.2114  grams  or  4.89  per  cent. 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SAI/TS 


1145 


whereas  another  sample  of  3.4729  grams  exposed  to  quiet  air  for  the 
same  time  lost  scarcely  a weighable  quantity. 

The  air  used  in  the  following  experiments  was  regulated  so  that  60-70 
bubbles  per  minute  passed  from  the  exit-tube  dipping  into  the  flasks 
■containing  the  standard  acid.  At  first  considerable  difficulty  was  en- 
countered in  regulating  this  passage  of  air  but  after  a number  of  trials 
the  following,  quite  satisfactory  system  was  adopted.  The  air  from  the 
main  supply  was  passed  through  a bottle  connected  on  the  one  hand  with 
the  drying-train  and  on  the  other  with  a shunt  tube  dipping  into  a defi- 
nite depth  of  water.  The  object  of  the  shunt  was  to  force  through  the 
drying  system  air  backed  by  a constant  pressure,  equal  always  to  the 
height  of  water  in  the  shunt  system  when  air  was  constantly  passing 
out  of  the  latter.  The  quantity  of  air  passing  through  the  drying  system 
was  controlled  by  a screw  clamp  attached  to  a rubber  tube  in  connection 
with  the  exit  tube.  By  this  means  the  number  of  bubbles  per  minute 
could  be  regulated  to  a nicety  ; the  size  of  the  bubbles  passing  through 

the  normal  sulphuric  acid,  was  limited,  of  course,  by  the  size  of  the  exit 

tube  dipping  into  it. 

Fifthly,  the  size  of  the  salt  crystals  used  was  found  to  exercise  a very 
appreciable  effect  on  the  results  ; mass  varying  as  the  cube,  and  radiat- 
ing surfaces  as  the  square  of  the  diameter.  To  reduce  this  influence  to 
a minimum  the  crystals  were  pulverized  so  as  to  pass  through  a particu- 
lar, fine-mesh  sieve. 

Sixthly,  efflorescence  of  some  salts,  for  instance  with 

ammonium  calcium  arsenate,  was  found  to  introduce  large 

factors  of  error.  The  weathering,  that  is  the  decomposition  of  these 
salts  at  ordinary  temperature,  is  often  so  great,  particularly  in  summer, 
that  only  freshly  prepared  samples  could  be  used. 

Finally,  the  manner  of  heating,  for  instance,  whether  in  open  crucible, 
in  desiccators  over  sulphuric  acid,  or  in  the  U tubes  mentioned  above, 
was  found  to  yield  different  per  cents,  of  decomposition.  Of  course  the 
same  method  of  heating  was  employed  throughout  any  given  experi- 
ment, nevertheless  at  some  temperatures  certain  abnormal  results  were 
often  obtained  and  can  be  explained  only  on  the  basis  of  “suspended 
transformation”.  For  instance  in  the  following  table  : 


Substance  Temperature  Per  cent,  volatilized 

A12(S01)3.i8H20 83.0  H-79 

“ 90-5  4-69 

KAl(S0,)2.i2H20 71-5  8.35 

“ 83.O  2.21 


it  is  seen  that  higher  temperatures  yield  lower  per  cents,  of  decompo- 
sition though  the  experiments  were  carried  out  under  similar  conditions. 
Heating  at  once  to  the  higher  temperatures  seems  to  induce  this  “sus- 
pended transformation.”1 
1 See  page  1 1 6 1 . 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


I 146 


Though  recognizing  the  above  mentioned  influences  and  observing  in 
the  experiments  every  precaution  necessary  to  avoid  their  disturbing' 
effects,  concordant  results  were  often  obtained  only  after  repeated  trials. 
This  difficulty  of  obtaining  concordant  results  was  particularly  notice- 
able at  temperatures  at  which  more  than  one  molecule  of  water  was  given 
off,  for  instance  in  the  case  of  ammonium  magnesium  arsenate,  at  70-90°. 

The  following  table  includes  data  obtained  in  studying  the  decompo- 
sition of  ammonium  magnesium  arsenate  prepared  in  the  usual  manner 
and  found  by  analysis  to  be  strictly  NH4MgAs04.6H20. 


I 

11 

III 

IV 

V 

VI 

VII 

VIII 

IX 

X 

XI 

XII1 

Tem- 

per- 

Weight 

salt 

Loss 

of  weight 

of 

Ratio 

of 

Total  per  cent,  loss 
of 

Fractional  per 
loss  of 

cent. 

ature 

HoO+NH; 

5 NH3 

HoO 

Both 

H0O+NH3 

NH3 

h2o 

H0O-FNH3  nh3 

h2o 

40 

O.7752 

.0008 

.0002 

.0006 

4.0 

O.  IO 

0.02 

0.08 

0.10 

0.02 

O.08 

45 

I.0344 

.0030 

.0003 

.0027 

9.0 

0. 29 

0.03 

0.26 

0. 19 

0.01 

O.18 

50 

O.2692 

.OC42 

.0003 

.0039 

13.0 

I.56 

O.II 

i-45 

I.27 

0.08 

1. 19 

56 

O.2875 

•0053 

.0005 

•CO53 

10.6 

2.01 

0.17 

1.84 

0-45 

0.07 

O.38 

60 

O.6945 

.0264 

.0027 

.0237 

8.8 

3.80 

0-39 

3-4i 

L79 

0.22 

1-57 

65 

O.6482 

.0629 

.0063 

.0566 

8.9 

9.70 

O.97 

8-73 

5-9° 

O.58 

5-32 

70 

O.8108 

.0828 

.OO93 

•0735 

8.0 

10.21 

I-I5 

9.06 

0.51 

0.18 

o-35 

75 

O.2698 

.0343 

.OO39 

.0304 

7.8 

12.71 

i-34 

11-37 

2.50 

O.I9 

2.31 

75 

O.2332 

.0379 

.OO38 

.0341 

8.9 

16.35 

1.72 

14.63 

3-64 

O.38 

3.26 

80 

O.5166 

.1563 

.OI64 

•1399 

8-5 

30.26 

3-i8 

27.08 

13-91 

I.46 

12.45 

82 

0.2402 

.0832 

.0087 

•0745 

8-5 

34-64 

3-63 

31.01 

4-38 

0-45 

3-95 

85 

O.3164 

.1162 

.0122 

.1140 

8-5 

36.72 

3-86 

32.86 

2.08 

0.13 

i-95 

100 

O.3247 

.1212 

.0127 

.1085 

8.6 

37.31 

3-9° 

33-41 

0-59 

0.04 

0.55 

no 

O.2920 

.IIO8 

.OIl6 

.0992 

8.6 

37-95 

3-97 

33.98 

0.64 

0.07 

o.57 

130 

0.0708 

.0288 

.0030 

.0258 

8.6 

40.68 

4.24 

36.44 

2-73 

0.27 

2.46. 

150 

O.II56 

.0478 

.0050 

.0428 

8.5 

41-35 

4-32 

37-03 

0.67 

O.08 

o-59 

170 

O.1478 

.0638 

N 

t". 

O 

O 

.0566 

8.0 

43-17 

4.86 

38.31 

1.82 

0-54 

1.28 

190 

O.2496 

.II06 

.0128 

.0978 

7.6 

44-32 

5-12 

39.20 

1-15 

0.26 

0.89 

210 

O.364I 

.1670 

.OI96 

.1474 

7-5 

45.86 

5-40 

40.46 

i-54 

0.28 

1.26 

225 

O.654I 

•3035 

.O384 

.2651 

6.9 

46.40 

5-9° 

40.50 

o-54 

0.50 

0.04 

When  NH4MgAs04.6H20  is  heated  it  may  sustain  any  of  the  follow- 
ing losses  or  their  intermediate  per  cents. : 

Loss  of  Molecules  of  Per  cent,  of  loss 


NH3 5.88 

h2o  6.23 

2H20  12.46 

3H20  18.69 

4H20  24.92 

5H20  31.15 

6H20  37-37 

6%H20 40.49 

6^H20  + NH3 --  46.37 


The  above  experimental  data  plotted  with  per  cents,  as  ordinates  and 
degrees  of  temperature  as  abscissas,  give  from  columns  VIII,  IX  and 
VII  respectively,  the  curves  of  evolution  of  ammonia,  water,  and  both 
ammonia  and  water. 

1 These  fractional  per  cents,  are  obtained  by  subtracting  adjacent  total  per 
cents.  ; they  indicate  the  effect  of  the  increment  of  temperature. 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


II47 


It  will  be  observed  that: 

1.  Both  water  and  ammonia  begin  to  be  given  off  at  40°  and  are  com- 
pletely removed  at  2250. 

2.  Fully  one-half  of  all  the  ammonia  is  expelled  below  8o°,  the  re- 
mainder, between  temperatures  85-225°. 

3.  About  the  same  ratios'  of  wTater  and  ammonia  are  given  off  at 
temperatures  60-2 io°2. 

1 Between  60-150°  the  ratio  of  weight  of  water  to  ammonia  averages  8.5  :i, 
which  is  equal  to  a molecular  ratio  of  8.0  :i.  The  mass  ratio  of  water  to  ammonia  in 
NH4MgAs04  is  6.9  : 1.  In  determining  the  data  for  the  above  table,  wide  variations 
from  the  ratio  of  8.5  :i  always  indicated  experimental  errors. 

2 See  column  VI  above.  This  approximate  constancy  of  ratio  is  not  in  evidence 
with  other  salts  (see  NH4MgP04.6H20)  except  at  high  temperatures. 


II48  WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 

4.  Water  is  gradually  given  off  below  65°. 

5.  Then  below  8o°  the  remainder  of  four  molecules  of  water  is  given 
off1. 

6.  The  next  two  molecules  are  given  off  at  temperatures  between 
80-150°. 

7.  The  last  one-half  molecule  of  water,  derived  from  the  ammonium- 
oxygen  group  (NH40-),  is  slowly  expelled  at  temperatures  1 50-225°;  at 
the  last  of  these  temperatures,  magnesium  pyroarsenate  is  formed. 

The  fact  that  two  molecules  of  water  are  given  off,  finally  and  inde- 
pendently of  each  other,  and  of  the  other  four  molecules  is  confirmed  by 
the  following  experiment.  The  salt  containing  the  six  molecules  of 
water  was  heated  for  three  hours  on  the  water-bath  in  closed  vessels  with 
a large  quantity  of  ordinary  alcohol.  After  cooling,  washing  by  decan- 
tation, first  with  alcohol,  then  with  ether,  and  finally  drying  for  a short 
time  in  a vacuum  dessicator,  it  was  found  that  dehydration  and  removal 


of  ammonia  from  the  salt  had  resulted. 

This  is  shown  in  the  following  analyses  : 

Per  cent,  loss 

0.3541  grams  substances  gave  0.2035  grams  Mg2As207  = 25.59 

0.4m  grams  substances  gave  0.3058  grams  Mg2As207  25-63 

Average 25.61 

Per  cent.  NH3 

0.0738  grams  substances  gave  0.0021  grams  NH3 2.85 

0.0524  grams  substances  gave  0.0015  grams  NH3 2.92 

Average 2.88 


Now  the  total  loss  by  ignition  less  the  ammonia  is  equal  to  the  water  , 
25.61-2.88=22.74  per  cent.  H20.  Theory  2HMgAs04.2H20  = 22.50 
per  cent.  H20.  Therefore,  after  dehydrating  NH4MgAs04.6H20  by 
means  of  ordinary  alcohol,  two  molecules  of  water  remain,  so  they  must 
be  different  from  the  other  four  molecules. 

This  difference  of  the  last  two  molecules  of  water  from  the  other  four 
molecules  evidently  must  involve  a difference  in  structure,  that  is,  there 
must  exist  for  these  water  molecules  different  forms  of  union  in  the 
parent  molecule.  If  it  is  tenable  that  such  differeyices  of  coherence  of  mole- 
cules of  water  involves  differences  of  structure,  then  conversely  it  may  be 
held  that  molecules  simultaneously  expelled  involve  similarity  of  structures. 
Now  since  it  is  true,  as  was  shown  above,  that  NH4MgAs04.6H20  pos- 
sesses two  molecules  of  water  differing  from  one  another  and  from  the 
1 It  may  appear  from  the  curve  of  evolution  of  water  that  five  and  not  four 
molecules  of  water  are  expelled  simultaneously;  but  it  must  be  remembered  that  each 
of  the  six  molecules  of  water  contributes  at  all  lower  temperatures  its  quota,  con- 
sequently at  the  decomposition  point  for  four  molecules,  a surplus  derived  from  the 
other  two  and  a half  molecules  will  be  obtained.  The  alcohol-dehydrating  method 
establishes  beyond  a doubt  the  dissimilarity  of  four  molecules  of  water,  (water  of 
crystallization)  from  the  remainder  of  water  (water  of  composition). 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SAETS 


1149 


other  four — it  is  interesting  to  see  what  structures  will  account  for  all  of 
the  facts. 

In  the  first  place  there  can  be  little  doubt  that  water  of  crystallization 
is  held  in  definite  molecular  structures,  and  that  the  structural  formula 
of  arsenic  acid  is  : 

(HO)s  = As  = 0 

and  that  its  ammonium  magnesium  salt  is  : 

/°\ 

Mg<(  />As  — O 
XCK  | 

O — NH4 

and  its  salt  containing  one  molecule  of  water  (water  of  composition)  is  : 

M g<  /As  = (OH)2 

o-nh4 

This  last  structure  accounts  for  the  fact  that  one  molecule  of  water  is 
given  off  finally  and  with  more  difficulty  than  the  other  five  molecules  of 
water.  The  fact  that  the  last  two  molecules  of  water  differ  from  the 
other  four  molecules  and  differ  from  each  other  may  be  accounted  for  by 
the  following  structure  : 

H — O — Mg  — O — As  EE  (OH)3 


o-nh4 

from  which  on  heating  one  molecule  of  water  would  certainly  be  more 
easily  expelled  than  the  other.1 

Now  since  there  are  four  hydroxyls  in  this  structure,  they  can  offer 
similar  points  of  attachment  for  four  molecules  of  water  (of  crystalliza- 
tion), as  may  be  seen  individually  in  the  structure  : 

H — O — H 

— 6 — H 

and  completely  in  the  structure  : 

HOH 


H — O — Mg  — O — - As  EE  ( — OH), 


HOH  ONH4 

1 Though  no  data  of  the  relative  stabilities  of  H3As04  and  Mg(OH)2  toward  heat 
are  available,  it  may  be  inferred,  since  the  former  shows  greater  tendency  than  the 
latter  to  add  water,  that  the  hydroxyl  attached  to  magnesium  is  more  easily  expelled 
than  the  hydroxyl  attached  to  arsenic.  However,  this  point  is  not  so  important, 
here,  as  the  establishment  of  the  structure  H — O — Mg  — O — As.  It  seems  reason- 
able to  hold  that  the  above  condition  of  magnesium  is  more  probable  than  as  shown 
XX 


in 


At  any  rate  the  above  structure  affords  the  necessary  number  of 


points  of  attachment  for  all  of  the  water  of  crystallization  and  accounts  for  the  con- 
stitution of  hydrated  ammonium  magnesium  arsenate  and  other  salts. 


ii5o 


WILLIAM  M.  DEHN  AND  EDWARD  O.  REUSE 


This  molecular  aggregate  could  split  off  four  molecules  of  water  at  or 
near  the  same  temperature;  at  a higher  temperature,  one  other  molecule 
of  water;  and  finally  and  with  difficulty  the  last  molecule  (and  a half)  of 
water. 

It  may  be  held  that  the  molecule  of  water  attached  to  the  oxygen  in 
H — O — Mg  differs  from  the  three  attached  to  the  oxygen  in — As — O — H 
and  consequently  could  involve  a difference  in  coherence  in  the  parent 
molecule.  That  this  molecule  actually  differs  is  shown  by  the  data  given 
on  page  1 161. 

It  is  observed  (see  plate  VI)  that  one  molecule  of  water  is  dissociated 
below  350,  hence  all  of  the  facts  are  in  harmony  with  the  above  structure. 

It  may  be  contented  that  ammonia  does  not  cohere  in  the  manner 
indicated  by  the  structure  — O — NH4  but  rather  in  the  manner  shown 
in  the  structure  — O — H.  Either  mode  of  union  is  in  harmony  with 

NH3 

the  main  structure  of  ammonium  magnesium  arsenate  as  shown  above, 
but  the  latter  of  these  two  perhaps  more  readily  accounts  for  the  ease 
with  which  ammonia  is  expelled  at  moderate  temperatures  simultaneous- 
ly with  the  water  of  crystallization.  However,  it  does  not  explain  the 
fact  that  the  last  portions  of  ammonia  are  expelled  with  difficulty  and 
long  after  all  of  the  “water  of  crystallization”  has  retired;  nor  does  it  ex- 
plain the  fact  that  different  salts  containing  the  same  mass  of  crystal 
water  manifest  varied  degrees  of  coherence  for  ammonia.  These  condi- 
tions can  be  explained  only  on  the  basis  of  composition  of  the  remainder 
of  the  compound,  that  is,  the  constituent  atoms  of  the  different  compounds 
possess,  either  individually  or  collectively,  varied  affinities  for  the  am- 
monia group,  and  for  some  of  the  water  molecules  (water  of  composi- 
tion). This  is  clearly  shown  in  the  following  studies,  wherein  (i)  the 
magnesium  atom  of  NH4MgAs04.6H.,0  is  substituted  by  calcium  and 
other  metals  and  (2)  the  arsenic  atom  is  substituted  by  phosphorus. 

Ammonium  Calcium  Arsenate. 

The  next  salt  studied  was  NH4CaAs04.6H20,  which  was  prepared  as 
follows  according  to  the  suggestion  of  Wach.  Triammonium  arsenate 
(1  part)  and  ammonium  chloride  (1  part)  were  dissolved  in  a little  water 
and  the  resulting  solution  was  treated  slowly  with  lime  water  as  long  as 
a precipitate  formed.  The  precipitate  consisted  of  glisteniug  white  crystals; 
after  filtering,  washing  with  alcohol,  then  with  ether,  and  finally  drying 
on  filter  paper,  they  were  obtained  free  from  traces  of  chlorine.  Analy- 
sis of  the  salt  gave  the  following  data  : 

0.3245  grams  substance  yielded  0.0827  grams  CaO  = 18.20  per  cent.  Ca. 

0.7757  grams  substance  yielded  0.0471  grams  NH3  = 5.61  per  cent.  NH4. 

0.1464  grams  substance  lost  0.0636  grams  at  I90°=43.45  per  cent.  NH3-f-H20 
0.8244  grams  substance  lost  0.3616  grams  at  200°=43.75  per  cent.  NH3+H20 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


115* 


Theory  Found  Wach  1 Bloxam2 


As04 37-70  ....  35.83  34.92 

Ca 18.36  18.20  17-52  17.29; 

NH4 5.90  5.61  5.37  5. 28: 

6H20  38.04  ....  41.15  ••  •• 

NH3  + 6^H20  43-93  43-75  ••  •■ 


Evidently  the  crystals  were  a purer  form  of  NH4CaAs04.6H20  than 
prepared  by  Wach  or  by  Bloxam.  They  weathered  rapidly  ; when  ex- 
posed to  air  (25°-35°)  for  four  hours  they  lost  2-20  per  cent,  in  weight. 

In  the  following  table  the  time  of  decomposition  in  each  Case  was  four 
hours  : 


I 

11 

III 

IV 

V 

VI 

VII 

VIII 

IX 

Temper- 

Weight 

Loss 

of  weight 

0 f 

Ratio 

Total 

per  cent. 

loss  of 

ature 

salt 

H2O+NH3 

nh3 

h2o 

of  both 

H0O+NH3 

nh3 

h2o 

28 

0.2093 

.0050 

2.40 

40 

0. 1854 

.0202 

.0006 

.0196 

32.7 

IO.90 

0-33 

10.57 

41 

0.1482 

.0202 

.0017 

.0185 

II. O 

I3-63 

1. 12 

12.51 

44 

O.1722 

.0396 

.OO32 

.0364 

II.4 

23.OO 

1. 91 

21.09 

45 

0. 1642 

.0602 

.0055 

.0547 

IO.  O 

36.66 

3.32 

33-34 

59 

0.1920 

.0690 

.0057 

.0633 

II. I 

35-94 

2.97 

32-97 

59 

0.1546 

.0562 

.0044 

.0518 

11. 8 

36.35 

2.84 

33-51 

70 

0. 1 240 

.0446 

.0035 

.0411 

11.7 

35-97 

2.82 

33-15 

80 

0. 1 798 

.0686 

.0054 

.0652 

12. 1 

38.15 

3-00 

35.15 

90 

0.1406 

.0564 

.0047 

•0517 

11. 0 

40.II 

3-34 

36.77 

100 

0. 1448 

.0590 

.0051 

.0552 

10.8 

40.85 

3-50 

37.30 

104 

0.1310 

.0540 

.0044 

.0496 

ir-3 

41.22 

3-37 

37.85 

1 10 

0. 1074 

.0440 

.0040 

.0400 

10. 0 

40.97 

3-72 

37.25 

130 

0.1516 

.0632 

.0059 

.0573 

9-5 

41.79 

3-90 

37.89 

150 

0.1448 

.0620 

.0056 

.0564 

10.0 

42.82 

3.86 

38.96 

170 

O.1713 

.0740 

.0067 

.0673 

10.0 

43.20 

3-94 

39.26 

190 

O.1394 

.0612 

.0060 

.0552 

8.0 

43-90 

4.29 

39.6l 

200 

0.8244 

.3616 

43-75 

Red  heat  0.4172 

.2048 

49.09 

“ “ 

0.3154 

•1536 

48.70 

When  NH4CaAs04.6H20  is  heated  it  may  sustain  any  of  the  follow- 
ing losses  or  their  intermediate  per  cents. 


Losses  of  Molecules  of  Per  cents,  of  loss 

NH3 5.57 

H20 5.90 

2H20 11.80 

3H20 .- 17-70 

4H20 23.60 

5H20 29.50 

6H20 35.40 

6^H20 38.36 

6^H20+NH3 43-93 


The  above  data  plotted  in  the  manner  of  Table  I gives  the  following 
curves. 

1 Schweigger’s  J.  chim.  phys.,  59,  288. 

2 Chem.  News,  54,  168. 


1152 


WILIvIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


The  same  misinterpretation  of  data  mentioned  in  connection  with  am- 
monium magnesium  arsenate  is  observed  with  the  calcium  salt.  Blox- 
am1  says  on  standing  36  days  in  the  air  it  loses  all  but  one  molecule  of 
water  ; Lefevre2  says  drying  at  ioo°  removes  all  but  one-half  a molecule 
of  water  ; Field3  says  it  becomes  anhydrous  at  140°  ; Kotschubey4  says  it 
retains  one  molecule  at  1250  ; and  Bloxam5  assigns  formulas  (As04)- 

1 Chem.  News  (1886)  54,  168. 

2 Ann.  chim.  phys.  [6]  27,  13. 

3 Jahrsb.  1858,  175.  , 

4 J.  pr.  Chem.,  49,  188. 

5 Chem.  News  (1886)  54,  169. 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SAETS 


1153 


Ca3NH4H2.3H20  and  (As04)6Ca6NH4H5.3H20  to  the  products  of  drying 
hi  vacuo  over  sulphuric  acid  and  drying  at  ioo°  respectively.  Failure 
to  recognize  the  fact  that  the  salt  begins  to  decompose  at  ordinary  tem- 
perature, and  loses  both  ammonia  and  water  at  higher  temperatures 
.accounts  for  these  inconsistencies. 

It  will  be  observed  that  : 

1.  Both  water  and  ammonia  are  given  off  more  easily  from  the  cal- 
•cium  salt  than  from  the  corresponding  magnesium  salt. 

2.  The  point  of  greatest  decomposition  is  40°-50°  with  the  calcium 
salt  instead  of  70°-8o°  as  with  the  magnesium  salt. 

3.  The  temperatures  at  which  all  ammonia  and  water  are  removed  is 
2250  with  both  salts. 

4.  The  calcium  salt  like  the  magnesium  salt  first  liberates  one  mole- 
cule of  water  then  simultaneously  three  molecules,  then  one  molecule  ; 
then  another : and  finally,  the  one-half  molecule  derived  from  the 
ammonium-oxy  group. 

Confirmation  of  the  fact  that  the  last  two  molecules  of  water  differ 
from  the  other  four,  is  secured  here  as  with  the  magnesium  salt  by  study- 
ing the  alcohol-dehydration  products.  The  calcium  salt  was  treated 
twice  with  alcohol  in  the  same  manner  as  with  the  magnesium  salt  ; it 
then  gave  the  following  analytical  data  : 

0.5274  grams  substance  yielded  0.3600  grams  Mg2As207 

0.5279  grams  substance  yielded  0.3670  grams  Mg2As207 

0.3274  grams  substance  yielded  0.0105  grams  NH3 

Theory  Found 

HCaAs04.2H20  NH4CaAs04.2H20  I II 

As  34.72  32.18  33.64  33.03 

NH3  0.00  7.32  3.20 

Evidently  the  salt  lost  part  of  its  ammonia  and  contained  just  two 
molecules  of  water.  All  of  the  evidence,  therefore,  seems  to  favor  a struc- 
tural formula  that  is  perfectly  analogous  to  that  of  NH4MgAs04.6H20, 
viz  : — 

H — O — H 

H — O — Ca  — O — As  = ( — 6 — h)3 

I 

H — O — H O — NH, 

Other  Alkali  Earth  Salts. 

By  treating  solutions  of  triammonium  arsenate  ( 1 part)  and  ammonium 
chloride  (1  part)  with  solutions  of  strontium  hydroxide  and  barium 
hydroxide  respectively,  in  exactly  the  same  manner  as  with  calcium 
hydroxide,  it  might  be  expected  that  the  analogous  compounds 
NH4SrAs04.6H20  and  NH4BaAs04.6H20  would  be  formed  ; the  precipi- 
tates resulting  were  found,  however,  to  contain  no  ammonia  and  onty  one 


i*54 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


molecule  of  water.  An  explanation  of  this  is  conceivable  when  it  is 
recalled  that  the  temperatures  of  maximum  decomposition  for  the  mag- 
nesium salt  is  70°-8o°;  for  the  calcium  salt,  40°-50°;  and  undoubtedly 
for  the  strontium  and  barium  salts,  below  the  temperature  of  formation 
(room  temperature  25°-30°). 

The  strontium  salt  prepared  in  the  manner  stated  was  light  and  fleecy 
while  in  suspension  and  powdery  when  dry.  By  dissolving  it  in  hydro- 
chloric acid,  adding  ammonia  to  incipient  precipitation,  filtering  and 
letting  stand,  the  solution  yielded  beautiful,  small,  transparent  crystals. 

I 1.0350  grams  powder  heated  at  350°  lost  o.  1170  grams 

II  0.8820  grams  crystals  heated  at  2250  lost  0.0965  grams 

Theory  Found 

HSrAs04.H20  I II 

i yz  H20  11. 00  11.29  10.94 

The  barium  salt  prepared  as  above  yielded  small,  pearly  crystals, 

0.6660  grams  substance  yielded  0.5257  grams  BaS04 

0.4365  grams  substance  heated  at  225 0 lost  0.0375  grams  H20 

Theorv  Found 

HBaAs04.H«0 


Ba  46.44  46.41 

1%  h2o s.59  8.60 


The  strontium  salt  of  the  above  composition  had  not  been  prepared 
hitherto,  but  undoubtedly  its  more  or  less  dehydrated  form  was  described 
by  Salkowski1,  Joly2,  Horman3,  Lefevre4  and  Schiefer0.  The  barium  salt 
is  described  : Berzelius  and  Mitscherlich  held  that  it  contains  one-half 
molecule  of  water,  the  others6  agree  that  it  contains  one  molecule  of 
water. 

These  salts  were  heated  in  open  crucibles  in  air  baths  and  yielded  the 
following  comparative  data  : 


HSrAs04.HoO 


Temperature 

Hours 

Weight 

Loss 

Loss  per  cent. 

Per  cent, 
of  total  H«0 

45° 

4 

O.II52 

.0026 

2.25 

20.45 

75° 

1 % 

I.3024 

.0621 

6.02 

54-72 

1250 

1 

I.4923 

.1085 

7.27 

66.10 

150° 

1 

1-5335 

.1151 

7.56 

68.72 

210° 

0.8612 

.0882 

10.24 

93-09 

225° 

1 

0.8496 

.0938 

11.04 

100.40 

350° 

Red 

1 

1.0350 

.1170 

11.29 

109.63 

Heat 

x/5 

1.0350 

.1470 

14. 28 

129.82 

1J.  pr.  Chem.,  1868,  148. 

2 Compt.  rend.,  104,  905. 

3 Inaug.  Diss.,  1879. 

4 Ann.  chim.  phys.,  (6)  27,  20. 

5 Zeitschrift  fur  die  gesammten  Naturw’ssenschaften  23,  364. 

6J.  pr.  Chem.,  49,  189;  Ibid.,  IO4,  139;  Ibid.,  40,  247,  Compt.  rend.,  58,  253; 
Lehrbuch  der  Chemie  von  Berzelius  ; Lehrbuch  der  Cliemie  von  Mitscherlich. 


decomposition  of  hydrated  ammonium  salts 


1155 


Per  cent,  of  total  HoO 

Loss 

HBaAs04.H20 

Loss 

Weight  Hours 

Temperature 

0.85 

percent. 

0.07 

.0008 

1. 0914  I 

60  0 

3.61 

O.31 

.0050 

I.6078  I 

115 

29-57 

2-54 

.03II 

1.2347  I 

135 

71-59 

6.15 

.0804 

I.3067  I 

150 

71.36 

,6.13 

.0986 

I.6078  I 

190 

95-34 

8.19 

.0966 

1-179° 

210 

99.88 

8.58 

.0643 

0.7496  1 

225 

I23.051 

10.57 

.1246 

1-1790  Vs 

Red 

When  these  salts  are  heated 

they  suffer  the  following 

Heat 

or  intermediate 

losses: 

h2o 

HSrAs04.H20 

HBaAs04.H20 

5.73 

i^H2Q 

8.59 

and  yield  the  corresponding  pyroarsenates.  It  will  be  observed  in  the 
above  table  that: 


1.  The  strontium  salt  gradually  loses  its  molecule  of  water  below 
1250;  the  barium  salt  loses  its  molecule  of  water  below  150°. 

2.  Both  salts,  like  the  magnesium  and  calcium  salts  studied,  lose  all 
water  at  2250. 

The  fact  that  the  arsenic  atom  has  no  affinity  above  2250  for  the  last 
hydroxyl  in  these  four  salts  studied  shows  that  there  must  exist  in  them 
the  same  structure:  AsEEO — H.  Furthermore  since  these  four  salts 

exhibit  little  difference  in  the  ease  of  dissociating  the  last  molecule  of 
water,  differences  that  can  be  attributed  to  variation  of  size  of  salt  crys- 
tals used,  etc.,  it  is  tenable  that  the  last  molecule  of  water  is  held  as 
shown  in  the  structure:  =As — (OH)3,  consequently  the  above-mentioned 
strontium  and  barium  salts  must  possess  the  following  structural  formulas: 
XX  /OH  /Ck  /OH 

Sr<  >As^OH  Ba<  >AsA}H 
XX  M3H  XK  XOH 

. Furthermore  it  may  be  gathered  from  the  above  experiments  that 
when  hydrated  ammonium  salts,  or  hydrated  salts  containing  no  ammon- 

1 When  ammonium  calcium  arsenate  and  these  two  salts  are  heated  in  crucibles  to 
temperatures  higher  than  2250,  say  by  igniting  to  a red  heat,  there  results  to  some  ex- 
tent the  following  reactions:  3M2As207  — 2M3(As04)2  -f  As205,  As205  = As203  + 02. 
Evidences  for  these  reactions  are  as  follows:  first,  the  residue,  easily  soluble  in  hy- 
drochloric acid,  shows  the  presence  of  arsenate,  but  no  arsenite;  secondly,  a sublimate 
obtained  on  heating  in  tubes,  yields  tests  for  both  oxides.  This  loss  with  the  calcium 
salt  was  mentioned  by  Wach2  and  Lefevre3;  the  second  reaction  is  described  by  Kopp1, 
the  stability  of  Ca3(  As04)2  toward  heat  is  confirmed  by  Simon5. 

2 Schwiegger’s  J.  Chem.  Physik.,  59,  265. 

3 Ann.  chim.  phys.,  (6)  27,  56, 

4 Ibid,  (3)  48,  106;  Jahrsb.,  1856,  385. 

5 Ann.  Physik.  ii  Chem.,  Pogg.,  40,  417. 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


II56 


ia  begin  to  decompose,  fractions  from,  each  molecule  of  water  ( and  of  the 
ammonia')  are  given  off  at  all  temperatures  lower  than  the  ones  at  which 
individually  they  are  completely  dissociated;  therefore,  for  instance  in 
NH4MgAs04.6H20,  from  which  1 mol.  NH3,  1 mol.  H20,  3 mols.  H20, 

1 mol.  H20,  and  1 mol.  H20  are  separately  dissociated,  all  of  the  higher 
dissociations  are  taking  place  fractionally  and  simultaneously  when  any  of 
the  lower  dissociations  are  taking  place  partially  or  completely , hence  with- 
in the  range  of  temperatures  of  dissociation,  no  definite  formula  can  be  as 
signed  to  the  residues. 

Salts  of  Phosphoric  Acid. 

It  was  shown  above  that  substitution  of  alkali  earths  elements  in 
NH4MgAs04.6H20  produced  complexes  more  easily  decomposed  than  the 
magnesium  salt;  it  now  remains  to  show  the  effect  of  substitution  of 
phosphorus  for  arsenic  in  NH4MgAs04.6H20.  It  was  found  as  a matter 
of  fact  that  this  salt,  though  more  stable  than  the  arsenic  salts,  decom- 
poses in  the  same  manner  and  consequently  must  possess  a similar  struc- 

H— O— H 


tural  formula:  H — O — Mg — P=( — O — H)3. 

: : I 

H— O— H O-NH, 

The  samples  of  NH4MgP04.6H20  used  were  found  by  ignition  to  be 
absolutely  pure;  and  in  the  following  experiments  were  heated  for  four 
hours,  except  fractions  indicated  by  temperatures  148-205°,  which  were 
heated  for  7 hours  each  (table  IV). 

When  heated,  NH4MgP04.6H20  may  sustain  any  of  the  following; 
molecular  loses  or  their  intermediate  per  cents  : 

Loss  ctf  Molecules  of  Per  cent,  of  loss 


nh3 

h.2o 

2H20 

3H20 

4H20 

5H20 

6H20 

6KH20 

6^H20  + NH3 


6- 93 

7- 34 

14.70 
22.04 
29-39 
36.74 
44.08 
47-34 

54.70 


It  is  seen  cm  comparing  Plates  II  and  IV  that: 

1.  Both  ammonium  magnesium  arsenate  and  ammonium  magnesium 
phqsphate  sustain  maximum  decompositions  between  70°  and  8o°. 

2.  The  former  begins  to  decompose  below  40°  ; the  latter  at  45°. 

3.  The  former  is  completely  decomposed  at  225°  ; the  latter  is  not 
completely  decomposed  at  360°. 

4.  Half  of  the  ammonia  of  the  former  is  liberated  below  8o°  ; half  of 
the  ammonia  of  the  latter  is  liberated  below  155°. 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


1157 


TABLE  IV 


I 

71 

in 

IV 

V 

VC 

VII 

VIII 

IX 

Temper- 

Weight 

Loss 

of  weight 

0 f 

Ratio 

Total 

per  cent. 

loss  of 

ature 

of  Salt 

H20-fNH3 

NHj 

h2o 

of  Both 

H2O  + NH3  nh3 

h2o 

40 

.5810 

.OOOO 

.OOOO 

.0000 

0.0 

0.00 

0.00 

0.00 

45 

.9808 

.0060 

.0005 

.0055 

II. O 

0.61 

O.05 

0.56 

50 

.4936 

.0120 

.OOO9 

.0111 

I2.3 

2.44 

O.18 

2.26 

55  . 

.3129 

.0108 

.0006 

.0102 

17.0 

3.26 

0.I9 

3-o  7 

60 

.5400 

.0193 

.0016 

.0177 

II. I 

3-57 

0.30 

3-27 

65 

.492 1 

.0247 

.0020 

.0227 

11 -3. 

5.02 

O.41 

4.61 

70 

•7473 

.0423 

.0042 

.O38I 

9.1 

5-66 

O.56 

5.10 

76 

.5140 

.1763 

.0030 

.1463 

48.8 

34.3o 

0-59 

33-71 

80 

.6504 

.2341 

.0040 

.23OI 

57-5 

36.00 

0.61 

35-39 

95 

.2254 

.0822 

.0016 

.0806 

50.4 

36.47 

O.71 

35-76 

no 

.4260 

•1557 

.0032 

•1525 

47-9 

36.55 

0-75 

35- 80 

127 

.1674 

.0624 

.0013 

.o6lI 

47-o 

37.28 

0-77 

36.51 

135 

.1249 

.0487 

.OOIO 

.0477 

47-7 

39.06 

0.80 

38.26 

148 

.1620 

.0664 

.0021 

.0643 

30.5 

40.98 

I.30 

40.68 

155 

.1126 

.0514 

.0040 

.0474 

11. 8 

45-65 

3-55 

42.10 

165 

.0926 

.0436 

.0041 

•0395 

9.6 

47.08 

4.42 

42.66 

175 

.0900 

.0436 

.OO43 

•0393 

9.1 

48-45 

4-77 

43-68 

186 

.0680 

•0336 

.OO33 

.0303 

9.2 

4941 

4.85 

44.56 

195 

.0526 

.0260 

.0026 

.0234 

9.0 

49-50 

4-94 

44-56 

200 

.1251 

.0624 

.0065 

•0559 

8.6 

49.88 

5-i9 

44.69 

205 

.1387 

.0702 

.0073 

.0629 

8.6 

50.61 

5.26 

45-35 

225 

•4734 

.2342 

•• 

50.53 

230 

•9374 

.4726 

50.52 

240 

•9374 

.4818 

51.39 

395 

•9374 

.4950 

52.50 

310 

.6631 

•3494 

52.70 

340 

.6631 

•3514 

53-oo 

360 

•3194 

.1704 

53-33 

Red  heat. 8724 

.4767 

54-63 

5.  With  both  salts,  first  one 

molecule 

of  water  is 

liberated 

; then 

simultaneously,  three  molecules  ; then  one  molecule;  again  one  molecule; 
and  finally  the  last  one-half  molecule1. 

When  NH4MgP04.6H20  was  heated  with  alcohol  in  the  manner  that 
the  corresponding  arsenic  salt  was,  similar  dehydration  and  removal  of 
ammonia  were  incurred.  After  a sample  was  heated  for  three  hours,  it 
gave  the  following  analytical  data  : 

0.4522  grams  lost  by  ignition  0.1460  grams  = 32.29  per  cent. 

0.4057  grams  lost  by  ignition  0.1230  grams  = 32.04  per  cent. 

Average  — 32. 16  per  cent. 

0.1651  grams  yielded  0.0026  gram  NH3  = 1.59  per  cent. 

Therefore  H20  = 32.16  — 1.59  = 30-57  per  cent. 

1 Here  as  with  the  corresponding  arsenic  salt,  Jive  molecules  of  water  are  ap- 
parently given  off  simultaneously  ; the  explanation  here  is  the  same  as  there. — The 
curve  here  shows  a liberation  of  one  molecule  of  water  below  67°.  This  harmonizes 
with  the  above  formula  wherein  the  molecule  of  water  attached  to  H — O— Mg — O — 
differs  from  the  other  three  molecules  of  “water  of  crystallization.” 


I 158  WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


Another  sample  was  heated  for  two  hours  with  alcohol  and  the  treat- 
ment with  alcohol  was  repeated  ; the  salt  then  gave  the  following 
analytical  data  : 

0.4539  grams  lost  by  ignition  0.3189  grams  = 29.74  Per  cent. 

0.4002  grams  lost  by  ignition  0.2811  grams  = 29.79  per  cent. 

Average  = 29.76  per  cent. 
0.0676  grams  yielded  0.0006  grams  NHS  = 0.89  per  cent. 

0.1201  grams  yielded  00011  grams  NH:5  — 0.95  per  cent. 

Average  = 0.92  per  cent. 

Therefore  H20  = 29.76  — 0.92  — 28.84  Per  cent. 
Theory  of  H20  in  HMgP04.2H20  ==  28.84  Per  cent. 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


1159 


Therefore,  upon  heating  ammonium  magnesium  phosphate  with  alcohol 
the  following  reaction  takes  place  : 

NIJ4MgP04.6H20  = HMgP04.2H20  + 4H20  + NH3 

Microcosmic  Salt. 

This  salt  yielded  comparative  data  with  greater  difficulty  than  any 
other  salt  studied.  The  cause  of  this  was  ultimately  found  to  be  owing 
to  variations  in  size  of  crystals  used.  After  pulverizing  and  passing  them 
through  the  sieve,  little  difficulty  was  encountered. 


I 

11 

hi 

IV 

V 

VI 

VII 

VIII 

IX 

X 

Temper- 

Weight 

Loss 

of  weight 

0 f 

Ratio 

Total  per  cent. 

loss  of 

ature 

Time 

of  Salt 

H20+NH3 

NHS 

HoO 

of  Both 

H0O  + NH3 

nh3 

h2o 

40 

4 

2.1818 

.0254 

.0008 

.0246 

30.7 

1. 16 

0.04 

1. 12 

45 

4 

1.3590 

.0498 

.0009 

.0489 

54-4- 

3.66 

0.07 

3-59 

50 

4 

O.9220 

.0788 

.OOO9 

.0779 

86.5 

8-55 

0.10 

8-45 

55 

4 

0.3876 

.0683 

.OOI3 

.0670 

5i.5 

17.62 

0.36 

17.26 

60 

4 

0.4848 

.1138 

.0020 

.IIl8 

55-9 

23.26 

0.43 

22.93 

64 

3 

0.4520 

.1226 

.0032 

.1194 

37-3 

27.13 

0.71 

26.42 

72 

4 

0.1205 

.0332 

.OOII 

.0321 

29.2 

27.61 

0.89 

26.72 

83 

3 

0-4439 

.1262 

.0054 

.1208 

22.3 

28.44 

1.23 

27.21 

88 

4 

0.8261 

.2404 

.0124 

.2280 

18.4 

29.IO 

1.50 

27.60 

104. 

4 

0.4764 

•1477 

.0105 

.1372 

13.0 

31.00 

2.20 

28.80 

1 3 6 

4 

0.2786 

.oSor 

.OO78 

.0723 

9-3 

32.75 

2.80 

29-45 

127 

4 

0.1898 

.0630 

.OO63 

.0567 

9.0 

33-29 

3-32 

29.97 

135 

4 

0.1656 

.0586 

.0071 

•0515 

7.2 

35-39 

4.29 

31.10 

148 

4 

0.2310 

.0840 

.0105 

•0735 

7.0 

36.46 

4-54 

31.92 

155 

4 

O.I219 

.0477 

.0064 

.0413 

6.4 

39-13 

5-49 

33.64 

165 

4 

0. 1664 

.0680 

.OO95 

•0585 

6.1 

40.86 

5-70 

35-i6 

175 

4 

0.1026 

.0422 

.OO58 

.0364 

6.3 

4I-I3 

5.65 

35.48 

186 

4 

0.0680 

Os 

00 

q 

.OO39 

.0250 

6.4 

42.50 

5-74 

36.76 

195 

4 

0.0818 

.0364 

.0050 

.0314 

6-3 

44-50 

6. 1 1 

38.39 

200 

4 

O.  IOl6 

•0459 

.0063 

.0396 

6-3 

45.18 

6.20 

38.98 

205 

4 

0.1648 

.0758 

.0122 

.0636 

5-2 

46.00 

7.40 

38.60 

240 

2 

0.6130 

.2235 

46.95 

295 

2 

I.2261 

.6154 

50.19 

310 

4 

1.0204 

.5152 

50.49 

360 

3 

0.5070 

.2558 

50.40 

Red  heat  1/5 

0.9328 

.4760 

51-02 

When  microcosmic  salt  decomposes  it  may  sustain  any  of  the  follow- 
ing or  intermediate  losses  : 


NH3 8.13 

H20. 8.61 

2H20 17.22 

3H20 25.83 

4H20 34.45 

5H20  43 -°5 

4H20+NH3 42.58 

5h2o+nh3...: 51.2a 


i i6o 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


It  is  observed  that  : 

1.  Three  molecules  of  water  are  expelled  simultaneously  below  62°. 

2.  The  fourth  molecule  of  water  is  expelled  below  1600. 

3.  The  last  molecule  of  water  here  as  with  ammonium  magnesium 
phosphate  is  not  completely  removed  at  360°. 

4.  The  ammonium  molecule  is  expelled  in  nearly  the  same  manner 
as  with  ammonium  magnesium  phosphate. 

All  of  the  facts  seem  to  support  the  following  structural  formula  for 
microcosmic  salt  : 


DECOMPOSITION  OF  HYDRATED  AMMONIUM  SALTS 


1 161 


HOH 

Na — Ov  :: 

>P  = (OH)s 
NH-CK 

which  upon  being  heated  liberates  first  the  three  molecules  of  water  held 
by  tetravalent  oxygen  ; then  the  molecule  derived  from  two  hydroxy 
groups ; and  finally  the  molecule  of  water  derived  from  the  ammonium* 
oxy  and  the  remaining  hydroxyl  group. 

Further  evidence  that  microcosmic  salt  contains  three  molecules  of 
water  that  are  similar  in  the  ease  of  dissociating,  and  consequently  that 
they  possess  similar  structural  attachments, isderived  by  the  alcohol-dehy- 
dration method.  After  heating  twice  with  alcohol  for  one  hour,  the 
residue  contained  9.60  per  cent,  ammonia  and  yielded,  by  ignition,  a 
loss  of  45.70  per  cent.,  or  a loss  of  only  a fraction  of  one  molecule  of 
water  had  been  sustained. 


Vapor  Pressure  Determinations. 


When  the  vapor  pressures  of  ammonium  magnesium  arsenate  are  care- 
fully determined  a distinct  break  in  its  curve  (vide  Plate  VI)  is  notice- 
able below  350,  indicating  the  liberation  of  one  molecule  of  water  at  this 
temperature.  The  following  data  represent  two  different  determinations: 


Series  I 

Temperature 

Pressure 

Temperature 

Pressure 

Temperature 

Pressure 

27 

O.61 

37 

6.96 

45 

II.70 

28 

O.92 

38 

7.62 

46 

13.08 

29 

I.23 

39 

7.65 

47 

13.80 

32 

2.48 

40 

7.67 

48 

14.50 

34 

4-36 

4i 

9.OI 

49 

15-90 

35 

5.65 

43 

9.70 

50 

17.30 

36 

6.30 

44 

IO.40 

5i 

19.40 

Series  II 

Temperature 

Pressure 

Temperature 

Pressure 

Temperature 

Pressure 

3i-5 

3.00 

35-5 

7-50 

44 

10.6 

32.0 

4.25 

40.5 

8.00 

49 

M-5 

33-o 

5-50 

42.0 

9-30 

54 

18.0 

When  this  salt  is  heated  at  once  to  temperatures  of  7o°-98°,  other  con- 
ditions remaining  the  same,  the  phenomenon  of  “suspended  transform- 
ation” becomes  manifest,  as  is  shown  in  Plate  VI  and  the  following 
data  : 


Suspended  Transformation 


Temperature 

Pressure 

Temperature 

30.0 

IO.70 

86. 

71.2 

29.02 

86.9 

78.0 

65.26 

88. 

79-0 

73.92 

89. 

80.3 

81.88 

89.4 

83.8 

IOI.75 

Pressure 

Temperature 

Pressure 

120.5 

90. 

186 

136.8 

90.5 

204 

150. 

91. 

225 

167. 

98. 

670 

177- 

98.7 

776 

1 162 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


Non-Suspended  Transformation 


Temperature 

Pressure 

Temperature 

Pressure 

Temperature 

Pressure 

45-5 

1.39 

72 

63.6 

88. 

364.5 

47-o 

6-34 

78 

148.7 

89 

404.8 

5°.  2 

7. 1 1 

79- 

169.0 

89.4 

417.0 

55-2 

9-38 

80.3 

I94.O 

90 

424.O 

60.6 

14.79 

81.4 

219.8 

91 

482.0 

64.0 

21.78 

83.8 

247-3 

92 

559-0 

70.5 

45-27 

86 

29  (.0 

98 

745-1 

71.2 

53-54 

87 

331-5 

Though  the  general  forms  of  these  two  curves  are  similar,  it  is  observed 
(1)  that  the  normal  pressures  are  not  exerted  when  the  salt  is  heated  at 


decomposition  of  hydrated  ammonium  salts  1163 

once  to  yo°  and  (2)  that  this  condition  of  suspended  transformation  is 
lost  at  990.  • Evidently  the  cause  of  this  condition  is  the  formation  of  a 
superficial  impervious  layer  of  dehydrated  substance  that  protects  the 
inner  portions  from  immediate  decomposition. 

The  vapor  pressures  of  the  other  curves  are  given  in  the  following 
tables  : 


Ammonium  Calcium  Arsenate1. 


Temperature 

Pressure 

Temperature 

Pressure 

Temperature 

Pressure 

31-5 

25.6 

44-0 

55-2 

62.7 

136.9 

34-o 

29-5 

46.5 

61.2 

65.I 

161.6 

39-4 

40.8 

5°.° 

73-6 

67.4 

188.3 

40.0 

45-3 

55.o 

93-6 

70.2 

222.7 

42.5 

50.5 

60.8 

124.5 

70.8 

246.3 

Ammonium  Magnesium  Phosphate1. 

Temperature 

Pressure 

Temperature 

Pressure 

Temperature 

Pressure 

28.0 

IO.36 

40.0 

12.82 

50.0 

16.54 

31-5 

IO.79 

42.5 

13.26 

55.0 

17.92 

34-0 

II.57 

44.O 

14.06 

58.5 

20.02 

39-4 

12.43 

46.5 

I5.27 

Vapor  Pressures2 

Temperature 

HNaNH4P04.4H20  NH4MgAs04.6H20  NH4MgP04.6H»0 

NH4CaAs04. 

30.6 

30.0 

32.8 

32.O 

19-5 

35-0 

14.5 

37-0 

25-3 

39-o 

12.0 

23.0 

47.0 

34-o 

42.2 

17.0 

54-3 

45-o 

45-o 

30.0 

61.5 

47-8 

30-3 

75-o 

65.6 

48.0 

33-o 

77.0 

69.0 

52.5 

44.0 

58.0 

... 

55-o 

45-5 

89.0 

106.0 

55-3 

95-5 

117.0 

56.8 

120.0 

141.0 

59-o 

65.O 

70.0 

132.5 

217.0 

6r.o 

73-o 

84.0 

151-0 

246.5 

64.5 

85.0 

85.0 

176.5 

390.0 

68.7 

113.0 

125.0 

215.0 

407.0 

70.8 

134.0 

137.5 

239.0 

71.2 

148.3 

154.0 

240.6 

75-o 

170.0 

212.0 

273.0 

78.0 

201.0 

267.0 

330.2 

82.5 

252.5 

405.0 

418.0 

1 All  vapor 

pressures  mentioned  thus  far 

were  determined  in  Dehn’s  tensim 

(This  Journal, 29, 1052.  )The  break  in  the  curve  represented  by  the  expulsion  one  of  mole- 
cule of  water  from  ammonium  magnesium  phosphate  is  not  plainly  marked  ; however 
the  vapor  pressures  here  were  not  determined  with  the  greatest  of  care. 

2 These  vapor  pressures  were  determined  in  the  Bremer- Frowein  form  of  tensi- 
meter  (Phys.  Chem.,  1,  5;  17,  52),  and,  being  only  approximately  correct,  are  useful 
here  only  in  showing  relative  pressures  at  higher  temperatures. 


1164 


WILLIAM  M.  DEHN  AND  EDWARD  O.  HEUSE 


Further  studies  by  the  methods  herein  expressed  are  being  made;  at 
present  one  may  safely  draw  the  following: 

Conclusions. 

1.  Hydrated  ammonium  salts  upon  being  partially  or  largely  dehy- 
drated yield  products  of  indefinite  composition,  for  the  reason  that 

2.  These  salts  at  elevated  temperatures  undergo  primary  or  secondary 
decompositions  of  all  of  the  different  dissociating  molecules  of  water  and 
ammonia,  consequently 


decomposition  of  hydrated  ammonium  saets  1165 

3.  Drying  on  the  water- bath  or  “drying  to  constant  weight”  cannot 
yield  homogeneous  products,  and  therefore, 

4.  Many  of  the  empirical  formulas  of  such  compounds  given  in  the 
literature  are  necessarily  incorrect. 

5.  The  affinity  and  manner  of  union  of  water  of  composition  do  not 
differ  largely  from  the  affinity  and  manner  of  union  of  ammonia. 

6.  Water  of  crystallization,  conforming  to  the  law  of  definite  propor- 
tions, must  be  held  in  definite  molecular  structures,  through  the  agency 
of  valency,  as  in  other  compounds. 

7.  Tetravalent  oxygen,  necessary  to  express  these  structures,  is 
loosened  at  temperatures  above  ioo°,  therefore  salts  usually  expell  water 
of  crystallization  below  this  temperature  and  water  of  composition  above 
this  temperature. 

8.  Finding  dissimilar  molecules  of  water  in  hydrated  salts,  leads  to  a 
conception  of  their  structure. 

Urbana,  Illinois. 

May  29,  1907. 


3 01 


12  072848614 


, 


. ' 


..V ’.'v: 


